High-strength steel member

ABSTRACT

A high-strength steel member having a predetermined chemical composition, having a tensile strength of 1,000 MPa or higher, containing 0.10% or more of, in terms of percent (%) by area, at least one Ti precipitate that has an average size of from 30 to 200 nm in terms of an average equivalent circle diameter and is selected from the group consisting of a Ti carbide, a Ti nitride, and a composite compound thereof, at a location of 1 mm in depth from a surface of the steel member, and containing 0.5 ppm by mass or more of non-diffusible hydrogen that is released in a temperature range of from 400 to 800° C. in a thermal desorption hydrogen analysis.

TECHNICAL FIELD

The present disclosure relates to a high-strength steel member.

BACKGROUND ART

Among steel members used for machines, automobiles, bridges, orbuildings, steel members that are required to have particularly highstrength are, for example, those in which chromium steels orchromium-molybdenum steels defined in JIS G 4104 or JIS G 4105 have beensubjected to a quenching/tempering treatment. Some steel members, suchas gears, are carburized and then quenched to have high strength.

Quenching is made after heating steel members to high temperatures atwhich austenite phases are formed. However, absorption of hydrogen intosteel members from atmosphere during heating may cause quenching crackafter quenching. Further, for example, a low tempering temperature offrom 150 to 200° C. as in the case of high-strength steel members maycause deterioration in ductility or toughness after tempering, sincehydrogen absorbed into the steel members during quenching is notsufficiently released by the tempering.

With regard to hydrogen embrittlement resistance of high-strength steelmembers (steel members having tensile strengths of 1,000 MPa or higher),for example, Patent Literature 1 describes that adding V, Nb and Ti tosteel to refine a prior austenite grain is effective for improvement indelayed fracture resistance.

Patent Literatures 2 to 4 each describe a technique of dispersing, insteel, a fine precipitate that exhibits a hydrogen trapping ability byhigh-temperature tempering after quenching, to improve delayed fractureresistance.

-   Patent Literature 1: Japanese Patent Application Laid-Open (JP-A)    No. H03-243745-   Patent Literature 2: JP-A No. 2000-26934-   Patent Literature 3: JP-A No. 2006-45670-   Patent Literature 4: JP-A No. 2001-288539

SUMMARY OF INVENTION Technical Problem

However, the conventional techniques described in Patent Literatures 1to 4 and the like have a limitation to fundamental improvement indelayed fracture resistance of, for example, high-strength steel membersthat are subjected to low-temperature tempering at from 150 to 200° C.after quenching.

An object in one aspect of the present disclosure is to provide ahigh-strength steel member excellent in delayed fracture resistance,which is one kind of hydrogen embrittlement resistance.

Solution to Problem

Solutions for achieving the object in one aspect of the presentdisclosure include the following aspects.

<1> A high-strength steel member having a chemical composition of, interms of percent (%) by mass:

C: from 0.10 to 0.50%,

Si: from 0.02 to 2.00%,

Mn: from 0.05 to 2.00%,

Cr: from 0.10 to 2.00%,

Ti: from 0.20 to 1.00%,

N: from 0.0020 to 0.0250%,

Al: from 0 to 0.100%,

V: from 0 to 0.50%,

Nb: from 0 to 0.50%,

Mo: from 0 to 1.00%,

B: from 0 to 0.0100%,

Cu: from 0 to 2.00%,

Ni: from 0 to 3.00%, and

a balance consisting of Fe and impurities,

having a tensile strength of 1,000 MPa or higher,

containing 0.10% or more of, in terms of percent (%) by area, at leastone Ti precipitate that has an average size of from 30 to 200 nm interms of an average equivalent circle diameter and is selected from thegroup consisting of a Ti carbide, a Ti nitride, and a composite compoundthereof, at a location of 1 mm in depth from a surface of the steelmember, and

containing 0.5 ppm by mass or more of non-diffusible hydrogen that isreleased in a temperature range of from 400 to 800° C. in a thermaldesorption hydrogen analysis.

<2> The high-strength steel member according to <1>, having a chemicalcomposition including, in terms of percent (%) by mass, one or more of

Al: from 0.005 to 0.100%,

V: from 0.01 to 0.50%,

Nb: from 0.01 to 0.50%, or

Mo: from 0.01 to 1.00%.

<3> The high-strength steel member according to <1> or <2>, having achemical composition including, in terms of percent (%) by mass,

B: from 0.0003 to 0.0100%.

<4> The high-strength steel member according to any one of <1> to <3>,having a chemical composition including, in terms of percent (%) bymass,

one or both of Cu: from 0.05 to 2.00% or Ni: from 0.05 to 3.00%.

<5> The high-strength steel member according to any one of <1> to <4>,wherein the Ti precipitate has an average aspect ratio of from 1.0 to3.0.

Advantageous Effect of Invention

According to one aspect of the present disclosure, a high-strength steelmember excellent in delayed fracture resistance, which is one kind ofhydrogen embrittlement resistance, can be provided.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 shows a schematic view for describing, in a case in which ameasurement object is larger than a “round rod steel having a size of 10mm in diameter (I)×50 mm in length L”, an extraction location of a testpiece when a test piece for measurement of a content of non-diffusiblehydrogen is extracted from the measurement object.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment that is an example of the present disclosurewill be specifically described.

In the present description, a numerical value range represented by“(from) . . . to . . . ” means a range that encompasses respectivenumerical values indicated before and after “to” as a lower limit valueand an upper limit value.

A content of each element in a chemical composition is expressed as anelement amount (for example, a C amount, a Si amount).

The indication “%” with respect to the content of each element in achemical composition means “percent (%) by mass”.

A high-strength steel member according to the present embodiment(hereinafter, also simply referred to as “steel member”) is a steelmember having a predetermined chemical composition and having a tensilestrength of 1,000 MPa or higher. The tensile strength of the steelmember is a value obtained by measurement according to JIS-Z 2241(2015).

The steel member according to the present embodiment contains 0.10% ormore of, in terms of percent (%) by area, at least one Ti precipitatethat has an average size of from 30 to 200 nm in terms of an averageequivalent circle diameter and is selected from the group consisting ofa Ti carbide, a Ti nitride, and a composite compound thereof, at alocation of 1 mm in depth from a surface of the steel member, andcontains 0.5 ppm by mass or more of non-diffusible hydrogen that isreleased in a range of from 400 to 800° C. in a thermal desorptionhydrogen analysis.

The steel member according to the present embodiment has theconfiguration described above and thus is a high-strength steel memberexcellent in delayed fracture resistance, which is one kind of hydrogenembrittlement resistance. The steel member according to the presentembodiment has been found as follows.

The inventors have specifically analyzed a delayed fracture behavior,which is one kind of hydrogen embrittlement phenomenon, using variousstrength of steel members that are produced by quenching/temperingtreatments.

It has been already found that, of hydrogen that has been absorbed froman external environment into a steel member, diffusible hydrogen thatdiffuses in the steel member at room temperature particularly causesdelayed fracture. The diffusible hydrogen can be measured from a curvehaving a peak at a temperature of about 100° C. in a “curve representingthe relationship between the temperature and the rate of hydrogenrelease from a steel member” obtained by heating the steel member at arate of 100° C./hour.

Accordingly, if hydrogen that has been absorbed from an externalenvironment is trapped at some portion in a steel member so as not to bediffused, it is possible to render hydrogen harmless and delayedfracture due to absorbed hydrogen is suppressed.

The presence of a trapping site of hydrogen (hereinafter, also referredto as “hydrogen-trapping site”) can be confirmed by comparing peaktemperatures and peak heights of hydrogen release curves obtained byheating, at 100° C./hour, a steel member before charging with hydrogenand a steel member after charging with hydrogen, respectively. Theamount of hydrogen trapped in a certain hydrogen-trapping site(hereinafter, also referred to as “hydrogen trapping capacity”) can bedetermined by an area integral value of the peak.

The inventors have performed the following evaluation with respect todelayed fracture resistance of a steel member that has been subjected tolow-temperature tempering at from 150 to 200° C. after quenching. A testpiece of a circularly notched rod steel having a diameter of 10 mm isheated for 20 minutes in an atmosphere of 1 atm containing from 30 to100% of hydrogen, quenched by cooling with water, and thereaftertempered at 150° C. for 30 minutes. A constant load (90% relative totensile strength) is applied to the test piece in the air and a time ismeasured until rupture is caused, thereby evaluating delayed fractureresistance. A longer rupture time means that the steel member is morefavorable in delayed fracture resistance.

As a result, the inventors have found that a steel member having a steelstructure containing 0.10% or more of, in terms of percent (%) by area,at least one Ti precipitate that has an average grain size of from 30 to200 nm and is selected from the group consisting of a Ti carbide, a Tinitride, and a composite compound thereof, at a location of 1 mm indepth from a surface of the steel member is excellent in delayedfracture resistance.

The steel member that has such a steel structure and is excellent indelayed fracture resistance exhibits, in a temperature range of from 400to 800° C., a hydrogen release peak which indicates that hydrogen stablytrapped in a hydrogen-trapping site consisting of the Ti precipitatedescribed above has been released, in the case of performing a thermaldesorption hydrogen analysis at a rate of 100° C./hour after the heattreatment under the conditions described above. The amount of hydrogenthat is released (hydrogen trapping capacity) is 0.5 ppm by mass ormore.

The inventors have made investigation by comparison with the “techniqueof dispersing, in steel, a fine precipitate that exhibits a hydrogentrapping ability by high-temperature tempering after quenching, toimprove delayed fracture resistance” described in Patent Literature 2(JP-A No. 2000-26934) and, as a result, have obtained the followingfinding. A fine Ti precipitate (at least one Ti precipitate selectedfrom the group consisting of a Ti carbide, a Ti nitride, and a compositecompound thereof) is precipitated at a higher temperature in the case ofcontaining a large amount of 0.20% or more of Ti. Therefore, it ispossible to cause precipitation during heating in quenching withouttempering, and hydrogen that has been trapped is released at a highertemperature. As can be seen therefrom, hydrogen is stably trapped, andthus hydrogen that has been absorbed from a heating atmosphere duringquenching can be trapped during cooling in quenching, thereby allowingfor rendering hydrogen harmless even in subsequent low-temperaturetempering. It has been thus found that delayed fracture resistance isexcellent as compared with the technique of Patent Literature 2.

It is noted that inclusion of an excess C causes deterioration indelayed fracture resistance. It is also noted that no inclusion of apredetermined amount of N causes generation of a coarse grain duringquenching and deterioration in delayed fracture resistance. Therefore, aC amount is set as from 0.10 to 0.50% and an N amount is set as from0.0020 to 0.0250%, as described below.

It has been found, based on the finding above, that the steel memberaccording to the present embodiment has the configuration describedabove and thus is a high-strength steel member excellent in delayedfracture resistance, which is one kind of hydrogen embrittlementresistance.

Further, a technique has been established which involves forming, in asteel member, a steel structure where “at least one Ti precipitateselected from the group consisting of a Ti carbide, a Ti nitride, and acomposite compound thereof” serving as a hydrogen-trapping site isfinely precipitated.

The reason why the steel structure at a location of 1 mm in depth from asurface of the steel member has been focused on is that delayed fracturedue to hydrogen embrittlement occurs at an inner portion that is at adepth of several hundred micrometers or more from the surface of thesteel member, originating at a site that is high in stress triaxiality.

Hereinafter, the steel member according to the present embodiment willbe specifically described.

(Hydrogen Trapping Capacity)

First, the reason for limiting the hydrogen trapping capacity (namely,the content of non-diffusible hydrogen) that is the most important forimprovement in delayed fracture characteristics of a high-strength steelmember is described.

Diffusible hydrogen that causes delayed fracture of a steel memberobtained by low-temperature tempering after quenching is absorbed intothe steel member from a heating atmosphere during quenching. Forexample, several ppm by mass of hydrogen is absorbed during heating toan austenite region, in a case of carburization quenching or quenchingwith heat by RX gas (endothermic converted gas) firing. Hydrogen in amartensite structure obtained by quenching is low in diffusioncoefficient, and thus hydrogen may be difficult to be completelyreleased by low-temperature tempering after quenching, possiblyresulting in hydrogen embrittlement.

When hydrogen is stably trapped in a hydrogen-trapping site duringheating in such an atmosphere and quenching, the content ofnon-diffusible hydrogen after quenching is increased, thereby allowingfor suppression of hydrogen embrittlement. In other words, hydrogen thatis released during re-heating to a temperature range of from 400 to 800°C. after quenching is in a form of hydrogen stably trapped in ahydrogen-trapping site, and is rendered harmless and does not contributeto hydrogen embrittlement.

Therefore, the steel member according to the present embodiment is asteel member containing 0.5 ppm by mass or more of non-diffusiblehydrogen that is released in a temperature range of from 400 to 800° C.in a thermal desorption hydrogen analysis. In other words, the hydrogentrapping capacity (content of non-diffusible hydrogen) is 0.5 ppm bymass or more.

The hydrogen trapping capacity is preferably 0.8 ppm by mass or more,and more preferably 1.0 ppm by mass or more, from the viewpoint ofimprovement in delayed fracture resistance. The content ofnon-diffusible hydrogen is preferably 3.0 ppm by mass or less, from theviewpoint of suppression of deterioration in forgeability due toincrease of a precipitate.

The steel member is controlled so as to have a steel structure in whichthe amount of non-diffusible hydrogen that is released in a temperaturerange of from 400 to 800° C. in a thermal desorption hydrogen analysisis 0.5 ppm by mass or more, thereby allowing delayed fracturecharacteristics to be improved.

The thermal desorption hydrogen analysis is performed as follows. First,a test piece of a round rod steel having a size of 10 mm in diameter(I)×50 mm in length L is taken from a steel member as a measurementobject. Next, the test piece is heated at 100° C./hour using a “gaschromatography type temperature rising hydrogen analysis apparatus” andthe amount (mass) of hydrogen that is released at each temperature isanalyzed.

A hydrogen release curve is thus obtained which exhibits therelationship between the temperature and the amount of hydrogen that isreleased. The amount of non-diffusible hydrogen that is released in atemperature range of from 400 to 800° C., namely, the hydrogen trappingcapacity (the content of non-diffusible hydrogen) is determined by anarea integral value of the peak in the hydrogen release curve.

In a case in which the measurement object is larger than the “round rodsteel having a size of 10 mm in diameter ϕ×50 mm in length L”, a testpiece obtained by scraping, from the measurement object, a “round rodsteel having a size of 10 mm in diameter ϕ×50 mm in length L”, whoseouter peripheral surface corresponds to a location at a depth of 1 mmfrom a surface of the measurement object, is used as the test piece (seeFIG. 1). In FIG. 1, OM represents the steel member as the measurementobject and SP represents the test piece.

In contrast, in a case in which the measurement object is smaller thanthe “round rod steel having a size of 10 mm in diameter ϕ×50 mm inlength L”, the measurement object is directly used as the test piece.This is because the content value of non-diffusible hydrogen that ismeasured does not vary even in a case in which the test piece is smallerthan the “round rod steel having a size of 10 mm in diameter ϕ×50 mm inlength L”.

It is noted that, even in a case in which the test piece is providedwith a circular notch, the content of non-diffusible hydrogen that ismeasured does not vary depending on presence or absence of the circularnotch.

(Steel Structure)

The steel member according to the present embodiment contains 0.10% ormore of, in terms of percent (%) by area, at least one Ti precipitatethat has an average size of from 30 to 200 nm in terms of an averageequivalent circle diameter and is selected from the group consisting ofa Ti carbide, a Ti nitride, and a composite compound thereof, at alocation of 1 mm in depth from a surface of the steel member. In otherwords, a volume (area) fraction of the Ti precipitate is 0.10% or morein terms of percent (%) by area.

The Ti precipitate has hydrogen trapping ability and serves as ahydrogen-trapping site that releases hydrogen at a relatively hightemperature of from 400 to 800° C. The presence of the Ti precipitatehaving hydrogen trapping ability enables non-diffusible hydrogen to bestably trapped during quenching of the steel member. In other words, thehydrogen trapping capacity (the content of non-diffusible hydrogen) of0.5 ppm by mass or more can be achieved. Thus, the steel member can beimproved in delayed fracture characteristics.

Ti oxides also have hydrogen trapping ability. However, it is preferablethat Ti oxides are not included in the steel member from the viewpointof securement of forgeability.

The Ti carbide, the Ti nitride, or the composite compound thereof(namely, Ti carbonitride) in the Ti precipitate is a compound thatmainly contains Ti as a metal component (Ti occupies 50% by atom or moreof a metal site) and has an FCC (face-centered cubic) structure.

From the viewpoint of increase in hydrogen trapping capacity andimprovement in delayed fracture characteristics, the volume (area)fraction of the Ti precipitate is preferably 0.10% or more, and morepreferably 0.20% or more, in terms of percent (%) by area. From theviewpoint of securement of toughness, the volume (area) fraction of theTi precipitate is preferably 1.00% or less, and more preferably 0.50% orless, in terms of percent (%) by area.

The volume (area) fraction of the Ti precipitate means a volume (area)fraction of the entire Ti precipitate contained in the steel member.

From the viewpoint of increase in hydrogen trapping capacity andimprovement in delayed fracture characteristics with securement oftensile strength, the average size of the Ti precipitate is preferably100 nm or less, and more preferably 80 nm or less, in terms of theaverage equivalent circle diameter. From the same viewpoint, the averagesize of the Ti precipitate is preferably 60 nm or more.

From the viewpoint of increase in hydrogen trapping capacity andimprovement in delayed fracture characteristics with securement oftensile strength, an average aspect ratio of the Ti precipitate ispreferably from 1.0 to 3.0. An upper limit of the average aspect ratioof the Ti precipitate is more preferably 2.0, and still more preferably1.5.

The volume (area) fraction of the Ti precipitate, the average size ofthe Ti precipitate (average equivalent circle diameter), and the averageaspect ratio of the Ti precipitate are each measured using a test pieceprepared by an extraction replica method, by means of a transmissionelectron microscope (TEM) equipped with an energy dispersive X-rayanalyzer (EDS). Specifically, the measurement is performed as follows.

A portion that is located 1 mm in depth from a surface of the steelmember (hereinafter, also referred to as a “measurement surface”) istaken from an arbitrary site of the steel member serving as themeasurement object, and a test piece is prepared by an extractionreplica method.

Next, an arbitrary region of the measurement surface of the test piece(region having a size of 5 μm×5 μm) is observed using TEM-EDS at amagnification of 30,000.

Next, a component of the precipitate present in the field of view forobservation is subjected to analysis of an electron beam diffractionpattern with TEM and analysis with EDS, thereby identifying the Tiprecipitate.

Next, an area ratio of the entire Ti precipitate present in the field ofview for observation is calculated.

The foregoing operation is performed five times, and the average valueof the resulting area ratio of the Ti precipitate is defined as thevolume (area) fraction of the Ti precipitate.

An equivalent circle diameter of the entire Ti precipitate present inthe field of view for observation is determined.

The foregoing operation is performed five times, and the average valueof the resulting “equivalent circle diameter” is defined as the averagesize (average equivalent circle diameter) of the Ti precipitate.

A long axis length and a short axis length of the entire Ti precipitatepresent in the field of view for observation are determined. The longaxis length of the Ti precipitate is defined as the maximum diameter ofthe Ti precipitate. The short axis length of the Ti precipitate isdefined as the maximum length of the Ti precipitate along with adirection perpendicular to the long axis.

The foregoing operation is performed five times, and the average valueof the resulting “aspect ratio (=ratio of long axis length and shortaxis length (long axis length/short axis length))” is defined as theaverage aspect ratio of the Ti precipitate.

The steel member according to the present embodiment preferably includesa refined prior austenite grain, from the viewpoint of improvement indelayed fracture characteristics.

A grain size of the prior austenite grain (hereinafter, also referred toas “prior γ grain size”) is preferably from 5 to 50 μm, more preferablyfrom 10 to 40 μm, and still more preferably from 15 to 30 μm, in termsof an equivalent circle diameter at a location of 1 mm in depth from asurface of the steel member.

The prior γ grain size is measured by the following method.

A portion that is located 1 mm in depth from a surface of the steelmember (hereinafter, also referred to as a “measurement surface”) istaken from an arbitrary site of the steel member serving as themeasurement object, and the measurement surface of the sample that hasbeen taken is subjected to embedded polishing, and thereafter etchedwith a picral solution (mixed solution of hydrochloric acid, picricacid, and alcohol) as a corrosive liquid. The measurement surface of thesample is imaged using an optical microscope (at a magnification of250), a γ grain boundary that has been imaged is subjected todigitization and binarization, a grain size of the prior γ grain ismeasured, and the average value thereof is determined.

(Chemical Composition)

The steel member according to the present embodiment preferably has achemical composition of, in terms of percent (%) by mass: C: from 0.10to 0.50%, Si: from 0.02 to 2.00%, Mn: from 0.05 to 2.00%, Cr: from 0.10to 2.00%, Ti: from 0.20 to 1.00%, N: from 0.0020 to 0.0250%, Al: from 0to 0.100%, V: from 0 to 0.50%, Nb: from 0 to 0.50%, Mo: from 0 to 1.00%,B: from 0 to 0.0100%, Cu: from 0 to 2.00%, Ni: from 0 to 3.00%, and abalance consisting of Fe and impurities, from the viewpoint ofimprovement in delayed fracture characteristics.

Al, V, Nb, Mo, B, Cu, and Ni in the chemical composition of the steelmember according to the present embodiment are optional components,namely, components that do not have to be included in the steel member.In cases in which these components are contained, each component ispreferably contained in an amount equal to or greater than the lowerlimit of the respective content amount of the component, which will bedescribed below.

C: from 0.10 to 0.50%

C is an element essential to secure the tensile strength of the steelmember (hereinafter, also referred to as “strength”). When the amount ofC is less than 0.10%, required strength cannot be obtained. In thisregard, when the amount of C is more than 0.50%, deterioration indelayed fracture resistance is caused as well as deterioration intoughness. Thus, the amount of C is from 0.10 to 0.50%. The amount of Cis preferably from 0.20 to 0.40% from the viewpoint of strength andtoughness.

Si: from 0.02 to 2.00%

Si has an effect of increasing the strength of the steel member by asolid-solution hardening action. When the amount of Si is less than0.02%, the action cannot be exerted. In this regard, when the amount ofSi is more than 2.00%, the action is saturated and it is not possible toexpect any effect commensurate with the amount. Thus, the amount of Siis 0.02 to 2.00%. The amount of Si is preferably from 0.20 to 2.00% fromthe viewpoint of exertion of the solid-solution hardening action.

Mn: from 0.05 to 2.00%

Mn is an element not only necessary for deoxidation and desulfuration,but also effective for improvement in hardenability for providing amartensite structure. When the amount of Mn is less than 0.05%, theeffect cannot be obtained. In this regard, when the amount of Mn is morethan 2.00%, a Mn precipitate is segregated in a grain boundary duringheating to an austenite region, thereby resulting in not onlyembrittlement of the grain boundary but also deterioration in delayedfracture resistance. Thus, the amount of Mn is from 0.05 to 2.00%. Theamount of Mn is preferably from 0.50 to 1.50% from the viewpoint ofimprovement in hardenability and delayed fracture resistance.

Cr: from 0.10 to 2.00%

Cr is an element effective for improvement in hardenability and increasein softening resistance during a tempering treatment. When the amount ofCr is less than 0.10%, the effect cannot be sufficiently exerted. Inthis regard, when the amount of Cr is more than 2.00%, deterioration intoughness and deterioration in cold workability are caused. Thus, theamount of Cr is from 0.10 to 2.00%. The amount of Cr is preferably from0.50 to 1.50% from the viewpoint of improvement in hardenability, andsuppression of deterioration in toughness and deterioration in coldworkability.

Ti: from 0.20 to 1.00%

Ti is an element that forms a fine Ti precipitate (at least one Tiprecipitate selected from the group consisting of a Ti carbide, a Tinitride, and a composite compound thereof) having hydrogen trappingability at a relatively high temperature of from 400 to 800° C., andthat contributes to improvement in delayed fracture resistance. Further,Ti has not only an effect of forming TiN during deoxidation and heattreatment, thereby preventing coarsening of an austenite grain, but alsoan effect of fixing N. When the amount of Ti is less than 0.20%, theseeffects cannot be exerted. In this regard, when the amount of Ti is morethan 1.00%, Ti cannot be melted even with heat during rolling and acoarse Ti precipitate remains, thereby resulting in adverse effect onmachinability or toughness. Thus, the amount of Ti is from 0.20 to1.00%. The amount of Ti is preferably from 0.30 to 0.80%, and morepreferably from 0.40 to 0.60%, from the viewpoint of formation of a fineTi precipitate, machinability, toughness, or the like.

N: from 0.0020 to 0.0250%

N is an element that forms a Ti nitride and contributes to improvementin delayed fracture resistance. N has an effect of forming nitrides ofAl, V, and Nb, thereby allowing refinement of a prior austenite grainand increase in yield strength. When the amount of N is less than0.0020%, these effects are less exerted. In this regard, when the amountof N is more than 0.0250%, these effect are saturated. Thus, the amountof N is from 0.0020 to 0.0250%. The amount of N is preferably from0.0030 to 0.0150%, from the viewpoint of improvement in delayed fractureresistance, refinement of a prior austenite grain, and increase in yieldstrength.

The chemical composition of the steel member according to the presentembodiment may include, in terms of percent (%) by mass, one or more ofAl: from 0 to 0.100%, V: from 0 to 0.50%, Nb: from 0 to 0.50%, or Mo:from 0 to 1.00%, and preferably includes, in terms of percent (%) bymass, one or more of Al: from 0.005 to 0.100%, V: from 0.01 to 0.50%,Nb: from 0.01 to 0.50%, or Mo: from 0.01 to 1.00%.

Al: from 0.005 to 0.100%

Al is an element that has not only an effect of forming AlN duringdeoxidation and heat treatment, thereby preventing coarsening of anaustenite grain, but also an effect of fixing N. When the amount of Alis less than 0.005%, these effects are difficult to be exerted. In thisregard, when the amount of Al is more than 0.100%, these effects areeasily saturated. Thus, Al is preferably from 0.005 to 0.100%.

V: from 0.01 to 0.50%

V is an element that is precipitated in combination with TiC and thatcontributes to fine dispersion of a precipitate. Further, V is anelement effective for forming a carbonitride, thereby refining anaustenite grain. It is noted that the effect is less exerted when theamount of V is not 0.01% or more, and is easily saturated when theamount of V is more than 0.50%. When the amount of V is more than 0.50%,workability is easily impaired due to increase in deformationresistance. Thus, the amount of V is preferably from 0.01 to 0.50%.

Nb: from 0.01 to 0.50%

Nb is an element that is precipitated in combination with TiC and thatcontributes to fine dispersion of a precipitate, as is the case with V.Further, Nb is an element effective for forming a carbonitride, therebyrefining an austenite grain. It is noted that the effect is insufficientwhen the amount of Nb is less than 0.01%, and is easily saturated whenthe amount of Nb is more than 0.50%. Thus, the amount of Nb ispreferably from 0.01 to 0.50%.

Mo: from 0.01 to 1.00%

Mo is an element that is precipitated in combination with TiC and thatcontributes to fine dispersion of a precipitate, as is the case with V.It is noted that the effect is insufficient when the amount of Mo isless than 0.01%, and is easily saturated when the amount of Mo is morethan 1.00%. Furthermore, when the amount of Mo is more than 1.00%,workability is easily impaired due to increase in deformationresistance. Thus, the amount of Mo is preferably from 0.01 to 1.00%.

The chemical composition of the steel member according to the presentembodiment may include B: from 0 to 0.0100% in terms of percent (%) bymass, and preferably includes B: from 0.0003 to 0.0100% in terms ofpercent (%) by mass.

B: from 0.0003 to 0.0100%

B is an element that suppresses grain boundary breakage and thatimproves delayed fracture resistance. Further, B is an element that issegregated in an austenite grain boundary, thereby resulting in aremarkable improvement in hardenability. It is noted that the effectsare difficult to be exerted when the amount of B is less than 0.0003%,and are easily saturated when the amount of B is more than 0.0100%.Thus, the amount of B is preferably from 0.0003 to 0.0100%. The amountof B is more preferably from 0.0003 to 0.0050%, from the viewpoint ofimprovement in hardenability and delayed fracture resistance.

The chemical composition of the steel member according to the presentembodiment may include one or both of Cu: from 0 to 2.00% or Ni: from 0to 3.00% in terms of percent (%) by mass, and preferably includes one orboth of Cu: from 0.05 to 2.00% or Ni: from 0.05 to 3.00% in terms ofpercent (%) by mass.

Cu: from 0.05 to 2.00%

Cu is an element effective for improvement in softening resistanceduring a tempering treatment. When the amount of Cu is less than 0.05%,the effect is difficult to be exerted. Further, when the amount of Cu ismore than 2.00%, hot workability is easily deteriorated. Thus, theamount of Cu is preferably from 0.05 to 2.00%. The amount of Cu is morepreferably from 0.05 to 1.00% from the viewpoint of suppression ofdeterioration in hot workability.

Ni: from 0.05 to 3.00%

Ni is an element for improvement in ductility that is deteriorated withincrease in strength. Further, Ni is an element for improvement inhardenability during a heat treatment to increase in tensile strength.When the amount of Ni is less than 0.05%, these effects are lessexerted. Further, when the amount of Ni is more than 3.00%, theseeffects are saturated and effects commensurate with the amount aredifficult to be exerted. Thus, the amount of Ni is preferably from 0.05to 3.00%.

The balance of the chemical composition of the steel member according tothe present embodiment consists of Fe and impurities.

The impurities refer to any components that are contained in a rawmaterial, or any components that are incorporated in the course ofproduction and are not intentionally contained. Further, the impuritiesalso encompass any components that are contained in amounts in rangesnot having any effect on properties of the steel member, even if thecomponents are intentionally contained.

Examples of the impurities include P and S. An amount of P and an amountof S are each preferably from 0 to 0.015%, for example, from theviewpoint of no effect on delayed fracture resistance. It is noted thatthe respective lower limits of the amount of P and the amount of S maybe more than 0% from the viewpoint of reduction in cost of removal of Pand cost of removal of S.

(Method of Producing Steel Member)

It is important for a method of producing the steel member according tothe present embodiment to include previously precipitating a Tiprecipitate that exhibits trapping ability in a rolling step duringproduction of a rolling steel member serving as a material of the steelmember, in order to address a variety of heat treatment conditionsduring production of the steel member.

For example, in a case in which a rolled bar steel member is used as thesteel member, a steel piece (billet) having the chemical compositiondescribed above is heated to a temperature of 1,250° C. or higher duringrod steel rolling to cause the Ti compound to become a solid solution,thereafter hot-rolled at a finish rolling temperature of from 900 to1,000° C., followed by being cooled to from 700 to 750° C. at an averagecooling rate of 40° C./s or less. Thus, an objective Ti precipitate canbe precipitated. In this case, the Ti precipitate is isotropicallyprecipitated.

The heating temperature of the steel piece (billet) refers to a surfacetemperature of the steel piece. The finish rolling temperature refers toa surface temperature of the rolled bar steel member immediately afterfinish rolling. The average cooling rate after finish rolling refers toa surface-cooling rate of the rolled bar steel member after finishrolling.

The rolled bar steel member in which the objective Ti precipitate hasbeen precipitated is heated to an austenite region (for example, 850 to1,050° C.), cooled to from 20 to 100° C. at a cooling rate of 40° C./sor less to be quenched, and subjected to low-temperature tempering at atemperature of from 150 to 200° C. for a duration of from 15 to 60minutes, thereby obtaining the steel member according to the presentembodiment.

Even in a case of employing a method of producing the steel memberwithout any rolling, such an objective Ti precipitate can be formed inthe steel member by appropriately controlling solid-solution andprecipitation of compounds.

EXAMPLES

Hereinafter, the present disclosure will be more specifically describedwith reference to Examples. It is noted that the respective Examples arenot intended to limit the scope of the present disclosure.

A test material having a chemical composition shown in Table 1 washeated to a temperature shown in Table 2, thereafter hot-rolled at afinish rolling temperature shown in Table 2, and cooled to 700° C. at anaverage cooling rate shown in Table 2 for rolling to a diameter ϕ of 20mm, thereby producing a circularly notched test piece (notch depth of 2mm, notch bottom radius of 0.25 mm, and notch angle of 60 degrees) madeof a round rod steel having a size of 10 mm in diameter ϕ×50 mm inlength L.

The test piece was heated in a carburizing heating atmosphere or in acondition simulating RX gas heating (1 atm, mixed atmosphere of 50% ofhydrogen and Ar, heating temperature of 1,000° C., and heating durationof 30 minutes), cooled to 20° C. with water at a cooling rate of 40°C./s or less to be quenched, and thereafter tempered at 150° C. for 20minutes.

Note that No. 28 as a comparative steel was tempered at 520° C. for 30minutes and No. 29 as a comparative steel was tempered at 400° C. for 40minutes.

The resulting test piece was subjected to measurement of the rupturetime with a constant load test at 3,000 kgf up to 100 hours. The tensilestrength was also measured.

Separately, the test piece immediately after tempered was subjected tothermal desorption hydrogen analysis and the hydrogen trapping capacityreleased at a temperature range of from 400 to 800° C. was measured, inaccordance with the method described above. The prior γ grain size, thevolume (area) fraction of the Ti precipitate, the average size of the Tiprecipitate (average equivalent circle diameter), and the average aspectratio of the Ti precipitate were also measured in accordance with themethods described above.

TABLE 1 Chemical composition (% by mass), balance = Fe + impurities No CSi Mn P S Cr Ti N Al B Others Note 1 0.41 0.20 0.80 0.010 0.005 1.210.55 0.0051 0.005 — 1.00Mo Example steel 2 0.20 0.19 0.76 0.008 0.0061.20 0.61 0.0152 — — — Example steel 3 0.18 0.15 0.75 0.006 0.006 1.500.21 0.0121 0.007 0.0012 — Example steel 4 0.10 0.02 1.05 0.006 0.0051.90 1.00 0.0123 0.005 — — Example steel 5 0.20 2.00 2.00 0.008 0.0041.17 0.62 0.0103 — — — Example steel 6 0.48 0.20 0.80 0.008 0.006 0.100.51 0.0084 — — — Example steel 7 0.22 0.15 0.77 0.007 0.004 1.00 0.620.0233 — — 0.3V Example steel 8 0.20 0.18 0.80 0.007 0.006 1.04 0.590.0152 — — 0.01Nb Example steel 9 0.18 0.20 0.76 0.007 0.004 1.20 0.200.0124 0.005 0.0003 0.10Cu Example steel 10 0.22 0.20 0.05 0.008 0.0041.38 0.51 0.0105 0.100 — — Example steel 11 0.20 0.19 0.75 0.008 0.0071.20 0.55 0.0151 — — 3.00Ni Example steel 12 0.10 0.10 1.00 0.006 0.0052.00 1.00 0.0103 0.005 — — Example steel 13 0.22 0.15 0.77 0.007 0.0041.01 0.62 0.0020 — — 0.01V Example steel 14 0.20 0.20 0.75 0.008 0.0041.19 0.61 0.0148 — 0.0100 — Example steel 15 0.11 0.19 0.72 0.007 0.0081.20 0.21 0.0119 0.006 0.0051 0.05Cu Example steel 16 0.40 0.20 0.800.010 0.005 1.20 0.50 0.0050 0.007 — 0.01Mo, 0.50V Example steel 17 0.190.22 0.75 0.008 0.004 1.18 0.55 0.0150 — — 2.0Cu Example steel 18 0.200.20 0.73 0.008 0.004 1.20 0.61 0.0155 — — 0.05Ni Example steel 19 0.200.18 0.80 0.007 0.004 1.05 0.60 0.0150 — — 0.5Nb Example steel 20 0.200.25 0.75 0.006 0.004 1.20 0.05 0.0150 — — — Comparative steel 21 0.410.20 0.82 0.010 0.005 1.17 0.00 0.0050 0.005 — 0.20Mo Comparative steel22 0.40 0.18 0.81 0.010 0.005 1.19 0.05 0.0052 0.008 — 0.20MoComparative steel 23 0.18 0.22 0.80 0.007 0.007 1.21 1.09 0.0050 — — —Comparative steel 24 0.60 0.21 0.83 0.008 0.004 0.03 0.50 0.0120 — — —Comparative steel 25 0.11 0.05 0.10 0.008 0.008 0.05 0.57 0.0152 — — —Comparative steel 26 0.20 0.20 0.75 0.008 0.004 1.20 0.55 0.0015 — — —Comparative steel 27 0.20 0.19 0.76 0.008 0.006 1.20 0.61 0.0152 — — —Comparative steel 28 0.36 0.16 0.97 0.008 0.008 — 0.29 0.0049 0.071 —0.05V, 2.93Mo Comparative steel 29 0.10 0.10 1.00 0.006 0.005 2.00 1.000.0103 0.005 — — Comparative steel

TABLE 2 Hydrogen trapping Production conditions capacity Ti precipitateFinish Average released at Volume Average size Characteristics Heatingrolling cooling from 400 to (area) (average Rupture time Tensiletemperature temperature rate 800° C. Prior γ fraction equivalent Aspectin constant strength No (°C.) (°C.) (°C./s) (ppm by mass) grain size (%)circle diameter) ratio load test (hr) (Mpa) Note 1 1280 950 20 0.83 20μm 0.32  60 nm 1.6 No rupture 1560 Example steel 2 1300 900 15 0.90 28μm 0.25 160 nm 1.3 No rupture 1350 Example steel 3 1265 925 20 0.53 31μm 0.18  90 nm 1.1 No rupture 1340 Example steel 4 1310 930 15 2.10 21μm 0.36 190 nm 1.4 No rupture 1235 Example steel 5 1290 900 25 1.05 33μm 0.28 100 nm 1.5 No rupture 1360 Example steel 6 1280 1000 40 0.88 30μm 0.31  90 nm 1.3 No rupture 1570 Example steel 7 1280 950 20 0.82 22μm 0.21  60 nm 1.6 No rupture 1370 Example steel 8 1280 950 30 0.95 18μm 0.28  70 nm 1.4 No rupture 1360 Example steel 9 1250 950 25 0.51 32μm 0.12  60 nm 1.2 No rupture 1330 Example steel 10 1270 950 25 0.95 22μm 0.28 100 nm 1.1 No rupture 1340 Example steel 11 1260 950 25 0.87 28μm 0.30  90 nm 1.6 No rupture 1350 Example steel 12 1290 950 25 2.05 22μm 0.26 190 nm 1.8 No rupture 1110 Example steel 13 1270 950 30 0.82 25μm 0.25  60 nm 1.5 No rupture 1320 Example steel 14 1270 950 12 0.90 33μm 0.26 160 nm 1.7 No rupture 1330 Example steel 15 1250 900 15 0.51 20μm 0.18  60 nm 1.3 No rupture 1310 Example steel 16 1265 920 30 0.80 18μm 0.41  60 nm 1.8 No rupture 1520 Example steel 17 1270 980 25 0.87 35μm 0.32  90 nm 1.3 No rupture 1360 Example steel 18 1280 950 30 0.90 36μm 0.33 160 nm 1.4 No rupture 1340 Example steel 19 1275 900 30 0.95 16μm 0.31  70 nm 1.3 No rupture 1330 Example steel 20 1260 930 25 0.03 35μm 0.05  10 nm 1.2 25 1330 Comparative steel 21 1260 920 25 0.00 22 μm0.00 — 1.3 4 1510 Comparative steel 22 1260 950 25 0.05 21 μm 0.04 20 nm 1.2 6 1520 Comparative steel 23 1250 970 25 0.18 35 μm 0.68310 nm 1.1 76 1350 Comparative steel 24 1260 920 30 0.81 37 μm 0.35 —1.6 32 1620 Comparative steel 25 1260 900 35 0.91 38 μm 0.38 100 nm 1.30  980 Comparative steel 26 1260 900 20 0.86 65 μm 0.25 120 nm 1.2 521350 Comparative steel 27 1115 920 16 0 21 26 μm 0.24 350 nm 1.6 83 1340Comparative steel 28 1260 920 25 0.13 36 μm 1.40  10 nm 5.4 32 1737Comparative steel 29 1250 945 24 1.84 21 μm 0.25 180 nm 1.9 0  960Comparative steel

In Tables 1 and 2, Nos. 1 to 19 correspond to Example steels and theothers correspond to Comparative steels. All of the Example steelsexhibited 0.5 ppm by mass or more of hydrogen trapping ability as shownin the Tables, and thus were found to be excellent in delayed fractureresistance.

On the contrary, Nos. 20, 21 and 22 corresponding to Comparative steelswere each an example where the Ti content was so low that the size ofthe Ti precipitate was small or no Ti precipitate was present, therebyresulting in a small amount of hydrogen trapping.

No. 23 corresponding to Comparative steel was an example where theamount of Ti was excess, by which a solid solution of TiC was notsufficiently formed and a coarse carbide was generated during heating inrolling, thereby resulting in a small amount of hydrogen trapping.

No. 24 corresponding to Comparative steel was an example where theamount of C was excess, thereby resulting in deterioration in delayedfracture resistance.

No. 25 corresponding to Comparative steel was an example where theamount of Cr was small and hardenability was insufficient, as a resultof which the steel was low in tensile strength after quenching and thuscould not withstand any load in the constant load test.

No. 26 corresponding to Comparative steel was an example where the steelwas small in amount of N, thereby resulting in generation of a coarsegrain during heating in quenching and deterioration in delayed fractureresistance.

No. 27 corresponding to Comparative steel was an example where theheating temperature during rolling was low, by which a solid-solution ofthe Ti compound was not sufficiently formed and a coarse Ti precipitatewas generated, thereby resulting in deterioration in delayed fractureresistance.

No. 28 corresponding to Comparative steel was an example where thetempering temperature was high, by which most of the Ti precipitate wasprecipitated during tempering and thus the size of the Ti precipitatewas small, thereby resulting in deterioration in delayed fractureresistance.

No. 29 corresponding to Comparative steel was an example where thetempering temperature was high, as a result of which the tensilestrength after tempering was low and thus the steel could not withstandany load in the constant load test.

It was thus found that the Comparative steels were low in delayedfracture resistance.

The disclosure of Japanese Patent Application No. 2017-123347 isincorporated herein by reference in its entirety.

All documents, patent applications, and technical standards describedherein are incorporated herein by reference to the same extent as ifeach individual document, patent application, or technical standard wasspecifically and individually indicated to be incorporated herein byreference.

1. A high-strength steel member having a chemical composition of, interms of percent (%) by mass: C: from 0.10 to 0.50%, Si: from 0.02 to2.00%, Mn: from 0.05 to 2.00%, Cr: from 0.10 to 2.00%, Ti: from 0.20 to1.00%, N: from 0.0020 to 0.0250%, Al: from 0 to 0.100%, V: from 0 to0.50%, Nb: from 0 to 0.50%, Mo: from 0 to 1.00%, B: from 0 to 0.0100%,Cu: from 0 to 2.00%, Ni: from 0 to 3.00%, and a balance consisting of Feand impurities, having a tensile strength of 1,000 MPa or higher,containing 0.10% or more of, in terms of percent (%) by area, at leastone Ti precipitate that has an average size of from 30 to 200 nm interms of an average equivalent circle diameter and is selected from thegroup consisting of a Ti carbide, a Ti nitride, and a composite compoundthereof, at a location of 1 mm in depth from a surface of the steelmember, and containing 0.5 ppm by mass or more of non-diffusiblehydrogen that is released in a temperature range of from 400 to 800° C.in a thermal desorption hydrogen analysis.
 2. The high-strength steelmember according to claim 1, having a chemical composition including, interms of percent (%) by mass, one or more of Al: from 0.005 to 0.100%,V: from 0.01 to 0.50%, Nb: from 0.01 to 0.50%, or Mo: from 0.01 to1.00%.
 3. The high-strength steel member according to claim 1, having achemical composition including, in terms of percent (%) by mass, B: from0.0003 to 0.0100%.
 4. The high-strength steel member according to claim1, having a chemical composition including, in terms of percent (%) bymass, one or both of Cu: from 0.05 to 2.00% or Ni: from 0.05 to 3.00%.5. The high-strength steel member according to claim 1, wherein the Tiprecipitate has an average aspect ratio of from 1.0 to 3.0.
 6. Thehigh-strength steel member according to claim 2, having a chemicalcomposition including, in terms of percent (%) by mass, B: from 0.0003to 0.0100%.
 7. The high-strength steel member according to claim 2,having a chemical composition including, in terms of percent (%) bymass, one or both of Cu: from 0.05 to 2.00% or Ni: from 0.05 to 3.00%.8. The high-strength steel member according to claim 2, wherein the Tiprecipitate has an average aspect ratio of from 1.0 to 3.0.
 9. Thehigh-strength steel member according to claim 3, having a chemicalcomposition including, in terms of percent (%) by mass, one or both ofCu: from 0.05 to 2.00% or Ni: from 0.05 to 3.00%.
 10. The high-strengthsteel member according to claim 3, wherein the Ti precipitate has anaverage aspect ratio of from 1.0 to 3.0.
 11. The high-strength steelmember according to claim 4, wherein the Ti precipitate has an averageaspect ratio of from 1.0 to 3.0.
 12. A high-strength steel membercomprising a chemical composition, in terms of percent (%) by mass: C:from 0.10 to 0.50%, Si: from 0.02 to 2.00%, Mn: from 0.05 to 2.00%, Cr:from 0.10 to 2.00%, Ti: from 0.20 to 1.00%, N: from 0.0020 to 0.0250%,Al: from 0 to 0.100%, V: from 0 to 0.50%, Nb: from 0 to 0.50%, Mo: from0 to 1.00%, B: from 0 to 0.0100%, Cu: from 0 to 2.00%, Ni: from 0 to3.00%, and a balance comprising Fe and impurities, having a tensilestrength of 1,000 MPa or higher, containing 0.10% or more of, in termsof percent (%) by area, at least one Ti precipitate that has an averagesize of from 30 to 200 nm in terms of an average equivalent circlediameter and is selected from the group consisting of a Ti carbide, a Tinitride, and a composite compound thereof, at a location of 1 mm indepth from a surface of the steel member, and containing 0.5 ppm by massor more of non-diffusible hydrogen that is released in a temperaturerange of from 400 to 800° C. in a thermal desorption hydrogen analysis.